Control behavior of an anti-locking brake system

ABSTRACT

To improve the control behavior of an anti-lock control system, in a method wherein the rotational behavior of the wheels is measured and evaluated and a vehicle reference speed, wheel slip data and wheel acceleration data and other control parameters are determined, the lowest wheel speed (v min ) is continuously compared with the highest wheel speed (v max ) during a braking operation. In the event of an approximate correlation between the lowest (v min ) and the highest (v max ) wheel speed and if the correlation lasts longer than a predefined length of time (T1, T2), the pressure in the brakes of the front wheels is permitted to increase or caused to increase after this length of time. The duration of the predetermined length of time (T1, T2) can be varied as a function of the acceleration or deceleration (v 3 , v 4 ) of the non-driven wheels.

BACKGROUND OF THE INVENTION

The present invention relates to a method of improving the controlbehavior of a brake system with anti-lock control, wherein therotational behavior of the wheels is measured and evaluation and logicalcombining of the wheel rotational speed data permits producing a vehiclereference speed, wheel slip data and wheel acceleration data and othercontrol parameters which are used to proportion and modulate the brakingpressure.

Electronically controlled anti-lock brake systems (ABS) have gained insignificance in the past years. Systems of this type are on the marketin large numbers and various designs. In the majority of systems knownin the art, the data required for anti-lock control are produced bymeasuring the rotational behavior of the vehicle wheels. A vehiclereference speed is determined by logically combining the signalsoriginating from the individual wheels and can be taken into account asa reference quantity to calculate the wheel slip and other controlparameters and, in addition, to set and modulate the braking pressure inthe wheel brakes of the individual vehicle wheels.

Difficulties are involved in the identification of the actual controlsituation by way of the data furnished by the wheel sensors and theadjustment or modulation of the braking pressure in terms of an optimalbraking behavior when the interpretation of the sensor signals is noabsolute indicator of the instantaneous driving and control situation.Further, per se contradicting demands in terms of wheel stability (bypressure relief) and a short stopping distance (by a maximum possiblebraking pressure) must be satisfied in pressure modulation which isperformed by the ABS controller on the basis of the so-called controlphilosophy, i.e., the predetermined algorithms, in response to therotational behavior of the individual wheels.

There is a number of situations which are especially difficult to beidentified by the ABS controller and which give rise tomisinterpretations if no special measures are taken. For example, bumpsand other disturbances in the road surface cause vehicle decelerationswhich are erroneously interpreted as locking tendency by the controller.Therefore, it is appropriate to analyze the rotational behavior of thewheels pursuant criteria of different types in order to discovercontradictions and to protect individual control measures, especiallythose which are critical in terms of safety, according to differentaspects and criteria.

An object of the present invention is to identify control situationswhere the braking pressure adjusted by the control is too low or isbelow the per se achievable optimum.

SUMMARY OF THE INVENTION

It has been found that this object can be achieved by a method includingthat the lowest wheel speed is continuously compared with the highestwheel speed during a braking operation, that in the event of acorrelation, at least by approximation, between the lowest and thehighest wheel speed and if the correlation lasts longer than apredefined time interval, the pressure in the wheel brakes of the frontwheels is permitted to increase, or caused to increase directly orindirectly, after this time interval. For example, the braking pressurein the front wheels is increased by opening of the inlet valves whichare inserted in the pressure fluid path from the master cylinder of thebrake system to the front-wheel brakes. The same effect can be achievedindirectly by rise of the reference speed which is used as a referencequantity for the proportioning of braking pressure.

In a preferred aspect of the present invention, the predefined timeinterval is varied as a function of the acceleration (or deceleration)of the non-driven vehicle wheels, and the predefined time interval isset to a small value as long as the acceleration of all non-drivenwheels is in excess of a predetermined limit value, and is set to alonger duration when the acceleration is lower (or the deceleration isgreater). Instead of the two-step method, it is also possible to arrangefor several steps, and the duration of the predefined time intervalsrises with decreasing acceleration.

The present invention is based on the consideration that a permanentstability of all wheels, i.e., a virtual correlation of the wheel speedsof the wheels, during a controlled braking operation is unusual and canbe assessed as an indicator of insufficient braking pressure which isbelow the optimum. Experience shows that during control of the brakingpressure proximate the wheel lock threshold, the brake slip ofindividual wheels rises to such an extent that the speed variation ofthe wheel concerned differs relatively greatly from the fastest wheel.This can be identified by constantly comparing the lowest with thehighest wheel speed. In the absence of such a discrepancy, or if thecorrelation lasts longer than a predefined time interval, this is anindicator of insufficient braking pressure. The front-wheel brakingpressure is increased directly or indirectly in this situation toimprove the control.

For example, counters can be used having count rates which predefine thetime intervals, in excess of which braking pressure increase isinitiated. These counters are reset as soon as the difference betweenthe lowest wheel speed and the highest wheel speed exceeds a predefinedlimit value of, for example, 95 to 98% or 96 to 97% of the highest wheelspeed.

Further features, advantages and possible applications of the presentinvention can be seen in the following description of further detailsmaking reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a simplified view of the essential components of a circuit forimplementing the method of the present invention.

FIG. 2 is a flow chart showing the individual steps in theimplementation of the method of the present invention.

FIG. 3 is a view of diagrams explaining the operation of a circuit ofthe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit for an electronically controlled anti-locksystem. The input data of the system are produced by way of wheelsensors S1 to S4. In a conditioning circuit 1, wheel speed signals v₁ tov₄ are produced on the basis of the data of the sensors which areavailable at the output of circuit 1 either as signals, or in the formof speed data when a data processing system is provided.

In a signal processing circuit 2, the time derivatives of the speedsignals v₁ to v₄ are produced in a known fashion, above all theacceleration signals v₁ to v₄, the jerk signals v₁ to v₄, and the slipsignals λ₁ to λ₄. A vehicle reference speed v_(REF) representative ofthe approximate vehicle speed is required to determine the wheel slip λ₁to λ₄.

The data produced by analysis of the sensor signals are sent to an ABSlogic 4 which generates braking pressure control signals bycomprehensive calculations on the basis of complex algorithms. Thepressure control signals control electrically operable hydraulic valvesor other actuators permitting adjustment and modulation of the brakingpressure in a hydraulic brake system or the brake force. The valve blockof a hydraulic brake system is assigned reference numeral 5 in FIG. 1.

The ABS logic 4 is frequently achieved by programmed circuits, i.e., byone or more microcomputers.

The basic circuits or program parts which are required to implement themethod of the present invention are incorporated in a speed andacceleration observer 6. This block is supplied with the wheel speeds v₁to v₄ and the acceleration signals v₃, v₄ as inlet signals, the indices3 and 4 referring to the non-driven wheels.

The lowest wheel speed v_(min) is continuously compared to the highestwheel speed v_(max) in the circuit block 6. The special features of themethod of the present invention become effective when the wheel speedsv₁ to v₄ during a controlled braking operation are very close to eachother, or practically hardly differ from each other, for predefined timeintervals which are in the order or some 100 msec. If the lowest wheelspeed v_(min) is in excess of a predetermined value k×v_(max) for theduration of the above predefined time intervals, and `k` ranges between96 and 97%, for example, this is an indication that the introducedbraking pressure might be below the optimal value.

If the lowest wheel speed v_(min) is almost coincident with the highestwheel speed v_(max) for longer than a defined time interval, thiscondition is signaled to the ABS logic 4 by the circuit 6, with theresult that the pressure is increased in the front-wheel brakes. Thetime of observation or the time interval until the reaction to theapproximate correlation of all wheel speeds is a function of theacceleration (a negative acceleration is also termed as `deceleration`)of the non-driven wheels during the braking operation. In a moderateretardation with a wheel acceleration in excess of roughly -0.5 g to-0.7 g (or a deceleration of 0.5 g to 0.7 g), the pressure in thefront-wheel brakes is already increased after a relatively short periodof 250 msec, for example, while a major retardation in excess of -0.7 gto -1 g requires a longer time of 400 msec, for example, to pass bybefore the circuit 6 causes braking pressure increase in the front-wheelbrakes by way of the ABS logic 4.

The functioning and operation of the speed and acceleration observer 6is shown in the flow chart of FIG. 2.

After the START of the subroutine in the embodiment of FIG. 2, it isinitially determined in the branching 7 whether the relation

    v.sub.min >k×v.sub.max, and k=0.96 to 0.97

is satisfied. If yes (i.e., all wheel speeds are very close to eachother), a counting action with a count rate R1 or R2 is triggered whichcorresponds to the predetermined time interval. As soon as a count Z_(G)is reached (branching 11), the braking pressure in the front-wheelbrakes is increased in step 12. This increase is achieved either bydirectly actuating and opening inlet valves, which are generallyinserted in the pressure fluid paths extending to the front-wheelbrakes, or by rise of the vehicle reference speed.

The time interval until the commencement of the braking pressureintroduction is predetermined by presetting the count rate R1 (programpart 9) or the count rate R2 (program part 10), namely, depending on themagnitude queried in branching 8 by which the non-driven wheels (wheels3, 4) are accelerated or decelerated. With a moderate acceleration whichexceeds an acceleration limit value, i.e.,

    v.sub.3,4 >a.sub.G

the count rate R1 applies. With a low acceleration (or greaterdeceleration) the count rate R2 applies. In one embodiment of thepresent invention, a_(G) =-(0.7 g to 1 g) applies.

Further, it can be seen from flow chart 2 that the counter R1, R2 isreset in step 13 when the condition

    v.sub.min >k×v.sub.max

is not satisfied, or when the count Z_(G) in step 11 is not reached.

The diagrams in the FIG. 3 embodiment are used to explain the describedconditions and the sequence run of the method of the present invention.FIG. 3A illustrates the operation of the counters with the count ratesR1 and R2 by which the predefined time intervals T1, T2 are achieved.The count limit value Z_(G) is achieved after 250 msec with count rateR1, but only after 400 msec with count rate R2. This is only an exampleof embodiment.

According to the FIG. 3B embodiment, the maximum and minimum wheel speedv_(max) and v_(min) differ only to a relatively small extent over a longperiod of time. The counter with count rate R1 is activated at time t₀,see FIG. 3C. At time t₁, the change in acceleration or the greaterdeceleration of the v_(min) -wheel results in a transition to the countrate R2. At time t₂, the relation

    v.sub.min <k×v.sub.max

k=0.96 . . . 0.97

applies. Consequently, the counter is reset by the operator 13 at timet₂ because the answer to the query 7 (FIG. 2) is `no`. Counting of thecount rate R1 is re-started at time t₂. The final count Z_(G) is reachedat time t₃, with the result that pressure increase in the front-wheelbrakes is initiated. The special program is thus completed. The programrun illustrated will restart.

Thus, according to the present invention, the braking effect is improvedin a very simple manner by introducing additional braking pressure in atransition condition where all wheels rotate at almost the same speedwith a moderate braking effect.

What is claimed is:
 1. Method of improving the control behavior of abrake system with anti-lock control in a two-axle vehicle with frontwheels and rear wheels, each provided with a wheel brake, wherein therotational behavior of the wheels is measured and evaluation and logicalcombining of wheel rotational speed data permits producing a vehiclereference speed, wheel slip data and wheel acceleration data and othercontrol parameters which are used to proportion and modulate a brakingpressure,wherein the lowest wheel speed is continuously compared withthe highest wheel speed during a braking operation, wherein, if acorrelation occurs, at least by approximation, between the lowest andthe highest wheel speed and if the correlation lasts longer than apredefined time interval, the pressure in the wheel brakes of the frontwheels is permitted to increase or caused to increase after this timeinterval.
 2. Method as claimed in claim 1,wherein the braking pressurein the front-wheel brakes is increased by opening inlet valves which areinserted in a pressure fluid path from a master cylinder of the brakesystem to the brakes of the front wheels.
 3. Method as claimed in claim1,wherein the braking pressure is increased by raising the value of thevehicle reference speed.
 4. Method as claimed in claim 1,wherein thepredefined time interval is varied as a function of the acceleration ofnon-driven wheels, and the predefined time interval is set to a smallvalue as long as the acceleration of all non-driven wheels is in excessof a predetermined limit value which is in the order of 0.5 g to -1 g,and is set to a longer duration when the acceleration is lower, i.e.when the deceleration is greater.
 5. Method as claimed in claim4,wherein the predefined time interval is varied in several steps, andthe duration of the predefined time intervals rises with decreasingacceleration, i.e. greater deceleration.
 6. Method as claimed in claim1,wherein counters are used having count rates which predefine the timeintervals, and a count rate in the order between 100 msec and 300 msecis preset at high wheel accelerations in excess of the limit value, anda count rate in the order between 300 msec and 500 msec is preset atlower accelerations, i.e. greater decelerations.
 7. Method as claimed inclaim 6,wherein the counters are reset as soon as the lowest wheel speedexceeds a predetermined limit value.
 8. Method as claimed in claim1,wherein a limit value for the lowest wheel speed of 95 to 98% of thehighest wheel speed is preset and wherein an approximation in excess ofthis limit value of the lowest wheel speed to the highest wheel speed isassessed as approximate correlation.